In recent years, the spread of thin-type TVs and laptops has been progressing, and the performance requirements for display devices such as a liquid crystal display, an organic EL display, and electronic paper have been desired to be higher. Furthermore, the display devices are further becoming finer, smaller, and thinner with the spread of high-function cellular phones and tablet-type terminal devices. A field effect transistor (Field Effect Transistor: FET) has been used for driving such display elements. Currently, FETs using silicon that is an inorganic material have been extensively used, however displays using an organic transistor element have been reported with a goal of reduction in cost, reduction in weight, and flexibilization, and the practical application of such displays has been expected.
However, in many of the displays using an organic transistor element, an organic field effect transistor (OFET) and a liquid crystal display part or electrophoretic cell are combined. It is difficult to obtain a large current from the OFET due to its structure and low mobility, and examples in which the OFET is used for a driver element of an organic EL display being a current-driven device that requires a large current have rarely been reported. Therefore, the development of an organic transistor element that is capable of driving an organic EL display and operates with a large current at a low voltage has been desired.
Currently, it is necessary that the channel length of the transistor element be made short in order to obtain a large current using the OFET, however it is difficult to make the channel length several μm or less with the patterning technology taking mass production into consideration. In order to solve the problem, studies on a “vertical type transistor structure” by which the operation can be performed with a low voltage and a large current by applying current in the direction of film thickness have been studied. The element that is used for a vertical type sandwiched device has, in general, a film thickness of several tens of nano meters to several hundreds of nano meters, and the control of the film thickness with a high accuracy of the order of a nanometer or less is possible. The short channel length of 1 μm or less can easily be achieved with the vertical type transistor by making a channel in the film thickness direction (vertical direction), and thereby there is a possibility that a large current is obtained. As such a vertical type organic transistor element, a vertical type transistor with a polymer grid triode structure using a self-organizing network structure of a polyaniline film as a grid electrode, a static induction transistor (Static Induction Transistor: SIT) that controls a current between a source and a drain by modulating depletion layer width with a finely striped intermediate electrode, and so on have been known so far.
Moreover, a technique that provides a vertical type organic transistor element exhibiting a high-performance transistor property by preparing an organic semiconductor/metal/organic semiconductor laminated structure has been proposed (Patent Literature 1). In the vertical type transistor element, an organic semiconductor layer and a striped intermediate metal electrode are provided between an emitter electrode and a collector electrode. In the organic transistor element, electrons injected from the emitter electrode transmit the intermediate metal electrode, thereby current modulation similar to that of a bipolar transistor can be observed, and since the intermediate metal electrode works like a base electrode, the organic transistor element is called a metal-base organic transistor (Metal-Base Organic Transistor: hereinafter, referred to as “MBOT”).
In the MBOT, almost no current flows when output voltage is applied between the emitter electrode and the collector electrode while voltage is not applied between the emitter electrode and the base electrode, however a current flows between the emitter electrode and the collector electrode when the voltage is applied between the emitter electrode and the base electrode. The current that flows between the emitter electrode and the collector electrode is a collector current, and the current that flows between the base electrode and the collector electrode is a base current. In the MBOT, the collector current is rapidly increased compared with the base current that is increased by applying a base voltage, and therefore the MBOT becomes an element with which the modulation of the collector current by the base voltage is made possible. A “leakage current” that flows when the voltage is applied between the emitter electrode and the collector electrode while the voltage is not applied between the emitter electrode and the base electrode is an OFF current, and the current that flows when the voltage is applied between the emitter electrode and the base electrode is an ON current. The MBOT is a transistor element in which the OFF current is nearly zero and with which a large ON current is obtained.
Moreover, as an example of a structure of the organic transistor (MBOT), an MBOT that can easily be produced by using a transparent ITO electrode as a collector electrode and laminating organic semiconductor/metal/organic semiconductor layers on the transparent ITO electrode by means of vacuum deposition has been reported (Patent Literature 2). In the MBOT, dimethyl perylene tetracarboxylic acid diimide (Me-PTCDI) and fullerene (C60) each being an n-type organic semiconductor material are used as an organic semiconductor, and, as electrode materials, Al (aluminum) is used as a base electrode and Ag (silver) is used as an emitter electrode. The MBOT becomes a transistor element in which the ON/OFF ratio (the ratio of the ON current to the OFF current) is improved, the transistor element capable of performing large current modulation by introducing a dark current suppression layer and applying heat treatment to the base electrode. As described here, the MBOT has a characteristic that a fine grid electrode and fine pattering of a stripe electrode are not necessary even though the MBOT is a vertical type transistor.
Moreover, as an example of the organic transistor element (MBOT), an MBOT (Patent Literature 3) having an organic semiconductor layer and a sheet-like base electrode between the emitter electrode and the collector electrode and having an energy barrier layer and a charge transmittance-promoting layer between the base electrode and the collector electrode, and further an MBOT (Patent Literature 4) in which an organic semiconductor layer comprising perylene tetracarboxylic acid diimide having a long chain alkyl group is provided on the side of the collector electrode to utilize as a collector layer have been proposed, and it has been reported that a favorable current modulation property and ON/OFF ratio can be obtained without applying heat treatment. Furthermore, an MBOT in which an organic semiconductor layer being present between the emitter electrode and the base electrode has a diode structure has been reported as a transistor element having a favorable amplification property (Patent Literature 5).
Moreover, as an example of the vertical type transistor, an element having an organic semiconductor layer between the emitter electrode and the collector electrode; a base electrode with concaves and convexes obtained by forming a comb-shaped aluminum layer on sheet-like aluminum; a pentacene layer formed between the emitter electrode and the base electrode; N,N′-diphenyl-N,N′-di(1-naphtyl)-1,1′-biphenyl-4,4′-diamine (NPB) formed as a hole injection layer; a thin film of lithium fluoride formed as a charge injection promoting layer; and a semiconductor laminated layer comprising pentacene and copper phthalocyanine between the collector layer and the base electrode has been reported as a transistor having a current amplifying property (Non Patent Literature 1 and 2).
Moreover, as an example of the vertical type transistor, a permeable metal substrate organic transistor having an organic semiconductor layer and a sheet-like base electrode between the emitter electrode and the collector electrode, the permeable metal substrate organic transistor utilizing a heterojunction organic semiconductor layer comprising N,N′-diphenyl-N,N′-di(1-naphtyl)-1,1′-biphenyl-4,4′-diamine (hereinafter, abbreviated as “NPB”)/fullerene (C60) for both of the organic semiconductor layer between the emitter electrode and the base electrode and the organic semiconductor layer between the collector electrode and the base electrode has been reported as a bipolar transistor (Non Patent Literature 3).
Moreover, as an example of the vertical type transistor, it has been reported that a vertical type transistor having an organic semiconductor layer and a comb-shaped base electrode between the L-shaped emitter electrode and the collector electrode in which vertical type transistor the organic semiconductor layer comprises BTQBT, [bis(1,2,5-thiadizolo)-p-quinobis(1,3-dithiol)], exhibits a large current value and ON/OFF ratio even though the organic semiconductor layer comprises a hole-transporting material (Patent Literature 6).
Moreover, there has been a proposal about an organic transistor element (MBOT) using a highly crystalline organic semiconductor layer between the emitter electrode and the collector electrode (Patent Literature 7). And in the literature, it is described that an MBOT using methylperylene that is an electron-transporting material exhibits a large current value, as large as 300 mA/cm2 and a high ON/OFF ratio, as high as 200, between the emitter and the collector with a low voltage by making a transistor element using the organic semiconductor layer with a crystal size being about the same thickness as the base electrode.